1. Field of the Invention
The present invention relates to a system in which a control device controls a controlled apparatus. It is particularly, but not exclusively, concerned with such a system in which the controlled apparatus is a rolling mill. The present invention relates to such a system, to a control device for such a system and also to a method of control using such a system.
2. Summary of the Prior Art
With increasing use of computerisation, devices for controlling a controlled system such as a rolling mill have become more complex. Such devices monitor the signals from the controlled apparatus, corresponding to operating conditions of that apparatus, and the devices apply appropriate control strategies.
In existing systems, the control devices are localised in that they only consider one, or a limited number, of parameters. Thus, the control operation carried out by one control device may then affect parameters controlled by another control device, leading to inefficient control.
Furthermore, although control strategies may be developed, the control by known controlled devices is relatively inflexible, and they cannot readily adapt to changing conditions in the controlled apparatus. Thus, to take the example of a rolling mill, steel sheets of a desired thickness are obtained by controlling the nip spacing between opposed rolls, and also by controlling the tension applied. However, changes occurring during the rolling process, and also changes in various mechanical conditions, make it difficult to establish satisfactory control. Furthermore, it can readily be appreciated that the various factors affecting the steel sheet are interrelated, so that control of one parameter will affect the control needed for another parameter.